With the widespread use of personal computers and the need to network them together, the ensuing volume of data traffic has accentuated the need for computer networks to operate at higher speeds. These speeds range from 10 Mbps (mega bits per second) to beyond 1000 Mbps. In light of the fact that the volume of data traffic is progressively increasing, network speed requirements well beyond 1000 Mbps may soon be required.
Standard high frequency data cable configurations typically utilize unshielded twisted pair (UTP) wiring in a four twisted pair configuration. These data cables are evaluated using several performance parameters. Three parameters of importance in this evaluation are impedance, attenuation and crosstalk. The Electronic Industries Association/Telecommunications Industry Association (EIA/TIA) provides standard specifications regarding the above-mentioned parameters in relation to attained transmission frequencies for data cable performance. These specifications are adopted throughout The United States of America as the standard for data cable performance. Moreover, in light of the domestic success of these cable standards, several foreign countries have adopted these or other similar standards.
As discussed above, three parameters of importance in evaluating data cable performance are impedance, attenuation and crosstalk. Impedance, in turn, is further categorized as characteristic or average impedance and input impedance (actual measured response). The characteristic or average impedance of twisted pair cables is primarily influenced by the dielectric constant of the material surrounding the conductor, the outside diameter of the insulated conductor and the outside diameter of the conductor itself. Theoretically, characteristic impedance is inversely proportional to the outside diameter of the conductor and the square root of the dielectric constant, and directly proportional to the distance between the centers of the conductors.
It has been found that the number of twists per foot in a twisted pair cable also has an impact on the impedance performance. The tighter the twist (or the more twists per foot), the lower the impedance performance, unless the effect is compensated by increasing the outside diameter of the insulated conductor. Impact on characteristic impedance due to pair twisting is believed to be caused by increased lay pitch influencing capacitive and inductive coupling between the conductors of a pair.
Input or actual measured impedance of a cable is largely influenced by conductor centering within its insulation, as well as conductor ovalness and insulated conductor ovalness. Secondary parameters affecting input impedance performance include insulation purity, pair-to-pair relationships, pair lay lengths (distance between successive twists), overall cable lay length and jacket tightness.
Conductor centering is measured, and expressed as a percentage, by dividing the minimum insulation wall thickness by the maximum wall thickness. This expression of centering assumes perfect ovalness of the copper and insulated wire. Ovality of the copper used in conductors is controlled by establishing stringent requirements and routine insulation tip and die inspection/maintenance schedules.
Another technique for controlling input impedance is to simultaneously extrude and bond the two insulated conductors of a pair in a single process. This approach, exemplified in U.S. Pat. No. 5,606,151, is aimed at assuring constant and consistent conductor to conductor spacing throughout the finished wire.
A disadvantage of using such a technique is that bonded pairs must be handled more carefully in further processing. Furthermore, bonded pairs limit the tightness of pair lays that can be utilized as well as overall production speeds at pairing. Another aspect of bonded pairs that is highly undesirable is the increased labor involved to install and terminate this product in a premises-cabling system. In order to install and terminate bonded pairs on data grade connecting hardware, the wires must first be separated. This step adds labor to installation and introduces a potential to performance degradation from human error if the wires are not evenly separated.
Yet another technique for controlling input impedance involves the use of planetary cabling or back twist pairing equipment utilizing back twist neutralizers. This approach actually creates a periodic inconsistency equal in length to the pairing lay length. Since most lay lengths in data grade (EIA/TIA 568-A Category 5) cables are less than 1.0", the influence of periodic inconsistencies on impedance performance will not be present at frequencies below 2 Gigahertz.
A disadvantage of such an approach is that planetary cablers can only operate at speeds of about 70 RPM (rotations per minute), significantly slowing the yield. For example, use of a planetary cabler operating at about 70 RPM with Category 5 pair lays of less than 1 inch, yields less than 6 feet per minute. Moreover, use of a back twist machine equipped with a back twist neutralizer induces hardening into the copper wire. The long term effect of copper work-hardening is an undesired feature. Twisted pair cables already exhibit a spring back effect due to the coiling and twisting of copper wires as the cable is produced. The use of a back twist neutralizer further work-hardens the copper and increases the overall spring back seen by installers of the finished cable.
Increase in network speed has also driven networking designers to switch from employing two pairs of a cable in half duplex (one pair in each direction) to using all four pairs operating in full duplex (all pairs in both directions). This has added an additional need to further control and specify input impedance to minimize signal reflections (return loss).
The second parameter of importance in evaluating data cable performance is attenuation. Attenuation represents signal loss or dissipation as an electrical signal propagates down the length of a wire. Attenuation is dependent on the dielectric constant and dissipation factor (loss tangent) of the insulating material surrounding a conductor, characteristic impedance of the wire and the diameter of the copper conductor.
According to the EIA/TIA 568-A standard, conductor size has to be in the range of 22 AWG (American wire gage)-24 AWG to work with standard based connecting hardware, while maintaining individual insulated conductor outside diameter of 0.048" or less and an overall cable outside diameter no greater than 0.250".
Dielectric constant and dissipation factor of the insulating material surrounding the conductor is dependent upon materials selected for the application. In case of twisted pair conductors, it is important to consider the effective dielectric constant. This is especially true at elevated frequencies (50 MHZ and higher) where the electromagnetic fields travel through a greater surrounding area as skin depths in the conducting material decrease with increasing frequency.
Attenuation is also influenced by input impedance. Input impedance fluctuations about the characteristic impedance value represent signal reflections (return loss). The percentage of reflected energy versus transmitted energy increases as frequency increases. It is due to this increase in reflected energy that it is possible to see spikes in attenuation loss curves, especially at frequencies in excess of 100 MHz. These spikes represent signal loss due to reflections. Reflections occur due to variations in the structure of a twisted pair that cause input impedance to deviate from its targeted characteristic value.
Dissipation factor or loss tangent is normally viewed as an insignificant contributor to signal loss until it exceeds 0.1. It is at this point (transition from a low loss dielectric to a lossy dielectric) when conductance becomes a significant factor in evaluating signal loss. The effect must be evaluated on a material by material basis to assure a stable low loss tangent throughout the frequency range and the temperature range the cable will be operated at. These values for determining the impact of the loss tangent are only guidelines and require interpretation, especially with UTP products operating above 100 MHz over lengths of 100 meters (attenuation is greater than 20 dB). The added loss due to dissipation factor properties of dielectric materials may become significant in calculating the total loss, even though the loss tangent may still be slightly less than 0.1.
The third parameter of importance in evaluating data cable performance is crosstalk. Crosstalk represents signal energy loss or dissipation due to coupling between pairs. The interaction between attenuation and crosstalk, i.e., attenuation-to-crosstalk ratio (ACR), provides a measure of cable performance. The greater the ACR, the more headroom or data capacity a cable has. While, near-end crosstalk (NEXT) is a measure of signal coupling between pairs when measured at the input end of the cable, far-end crosstalk is a measure of signal coupling between pairs when measured at the output end of the cable.
Theoretically, crosstalk is proportional to the square of the distance between conductor centers of the energized pair and inversely proportional to the square of the distance between the center point of the energized pair and the receiving pair. Crosstalk coupling between pairs is also inversely proportional to the dielectric constant of the material separating the two pairs. Dissipation factor can also influence the amount of energy coupled between pairs, provided there is significant pair-to-pair separation and a relatively lossy material (loss tangent&gt;0.1) is employed. However, a lossy material generally results in degraded attenuation performance, so the materials position with respect to the conducting pair must be considered.
EIA/TIA standards, however, only provide specifications for the above mentioned parameters, i.e., impedance, attenuation and crosstalk, in relation to transmission frequency up to 100 MHz. In particular, EIA/TIA 568-A for Category 5 cables regulates the performance of data cable up to a transmission frequency of 100 MHz. In addition to impedance, attenuation, and crosstalk, the EIA/TIA 568-A standard specifies dimensional constraints that must be adhered to by cable manufacturers when manufacturing high frequency data cables. For example, the EIA/TIA 568-A standard specifies that the conductor size fall within 22-24 AWG, the maximum insulated outside diameter be 0.048" and the maximum cable outside diameter (including jacket) be 0.250".
Realizing the need to provide data cable capable of achieving higher transmission frequencies, several manufacturers are attempting to produce cable that purportedly can achieve transmission frequencies in excess of 100 MHz. However, such data cables do not follow any guidelines beyond those set forth by the EIA/TIA 568-A Category 5 standard for transmission frequencies up to 100 MHz.
Any high performance data cable that performs at transmission frequencies in excess of 100 MHz, must meet or exceed the minimum impedance, attenuation and crosstalk parameters set forth for transmission frequencies up to 100 MHz by the EIA/TIA standard. Aside from electrical requirements, the EIA/TIA standard also sets forth physical requirements for the cable, e.g., conductor size, maximum insulated outside diameter, and the maximum cable outside diameter. However, as mentioned before, the EIA/TIA standard does not address requirements beyond the transmission frequency of 100 MHz.
It is therefore an object of the present invention to provide a high performance data cable that accommodates future growth in network speeds while meeting or exceeding the minimum impedance, attenuation and crosstalk parameters set forth for transmission frequencies unto 100 MHz by the EIA/TIA 568-A standard.
It is another object of the present invention to provide a high performance data cable that accommodates future growth in network speeds while satisfying the dimensional requirements set forth in the EIA/TIA 568-A standard.
It is yet another object of the present invention to provide a standard for a high performance data cable having a highest test frequency of 400 MHz.
It is a further object of the present invention to provide a high performance data cable that accommodates future growths in network speeds by controlling impedance, attenuation and near-end crosstalk.